The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, including the two vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints. The disc consists of the end plates at the top and bottom of the vertebral bones, the soft inner core, called the nucleus and the annulus fibrosis running circumferentially around the nucleus. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. A normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosis. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus on its own is not very effective in withstanding compression and shear forces.
As people age the intervertebral discs often degenerate naturally. Degeneration of the intervertebral discs may also occur in people as a result of degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding their increased share of the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones.
As the annulus loses stability it tends to bulge outward and may develop a tear allowing nucleus material to extrude. Breakdown products of the disc, including macroscopic debris, microscopic particles, and noxious biochemical substances build up. These breakdown products stimulate sensitive nerve endings in and around the disc, producing low back pain and sometimes, sciatica. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted.
Degeneration of a disc is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain.
Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: heating the annular region to destroy nerve endings and strengthen the annulus; applying rigid or semi-rigid support members on the sides of the motion segment or within the disc space; removing and replacing the entire disc with a generally rigid plastic, articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebrae adjacent the affected disc.
Until recently, spinal fusion has generally been regarded as the most effective surgical treatment to alleviate back pain due to degeneration of a disc. While this treatment is often effective at relieving back pain, all discal motion is lost in the fused spinal motion segment. The loss of motion in the affected spinal segment necessarily limits the overall spinal mobility of the patient. Ultimately, the spinal fusion places greater stress on the discs adjacent the fused segment as these segments attempt to compensate for lack of motion in the fused segment, often leading to early degeneration of these adjacent spinal segments.
Current developments are focusing on treatments that can preserve some or all of the motion of the affected spinal segment. One of these methods to stabilize the spinal motion segment without the disadvantages of spinal fusion is total disc replacement. Total disc replacement is a highly invasive and technically demanding procedure which accesses the disc from an anterior or frontal approach and includes dividing the anterior longitudinal ligament, removing the cartilaginous end plates between the vertebral bone and the disc, large portions of the outer annulus and the complete inner nucleus. Then an artificial total disc prosthesis is carefully placed in the evacuated disc space. Many of the artificial total disc prosthesis currently available consist of a generally rigid plastic such as ultra high molecular weight polyethelyene (“UHMWPE) as the nucleus that is interposed between two metal plates that are anchored or attached to the vertebral endplates. A summary of the history of early development and designs of artificial discs is set forth in Ray, “The Artificial Disc: Introduction, History and Socioeconomics,” Chpt. 21, Clinical Efficacy and Outcome in the Diagnosis of Low Back Pain, pgs. 205-225, Raven Press (1992). Examples of these layered total disc replacement devices are shown, for example, in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and 6,533,818.
These types of artificial total discs have several disadvantages. First, because the artificial disc prostheses are relatively large, they require relatively large surgical exposures to accommodate their insertion. The larger the surgical exposure, the higher the chance of infection, hemorrhage or even morbidity. Also, in order to implant the prosthesis, a large portion of the annulus must be removed. Removing a large portion of the annulus reduces the stability of the motion segment, at least until healing occurs around the artificial disc. Further, because the devices are constructed from rigid materials, they can cause serious damage if they were to displace from the disc space and contact local nerve or vascular tissues. Another disadvantage is that rigid artificial disc replacements do not reproduce natural disc mechanics.
An alternative to total disc replacement is nucleus replacement. Like an artificial disc prosthesis, these nucleus replacements are also inert, non-biological prostheses. The procedure for implanting a nucleus replacement is less invasive than the procedure for a total disc replacement and generally includes the removal of only the nucleus and replacement of the nucleus with a prosthesis that may be malleable and provide cushioning that mimics a natural disc nucleus. Examples of the prostheses used for nucleus replacement include: U.S. Pat. Nos. 4,772,287, 4,904,260, 5,192,326, 5,919,236 and 6,726,721.
Nucleus replacements are intended to more closely mimic natural disc mechanics. To that end, some nucleus replacements utilize hydrogels because of their water imbibing properties that enable these replacements to expand in situ to permit a more complete filling of the evacuated nucleus cavity. However, there is usually a trade-off in that the more expansion the hydrogel achieves, the less structural support the end product can provide. As a result, many hydrogel nucleus disc replacements have generally adopted the use of some form of a jacket or fabric to constrain the hydrogel material. For example, the implant described in U.S. Pat. Nos. 4,772,287 and 4,904,260 consists of a block of hydrogel encased in a plastic fabric casing. The implant described in U.S. Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabric shell. Without the jacket or other form of constraint, the hydrogel is susceptible to displacement because of the slippery nature of the hydrogel. Unfortunately, the jacket or fabric shell will be subject to long term abrasive wear issues that could result in failure of jacket or shell's ability to constrain the hydrogel and thus the hydrogel may be subject to displacement.
Another approach to nucleus replacement involves implantation of a balloon or other container into the nucleus, which is then filled with a biocompatible material that hardens in situ. Examples of this in situ approach to nucleus replacement include U.S. Pat. Nos. 6,443,988 and 7,001,431. One of the problems with this approach is that the chemical hardening process is exothermic and can generate significant amounts of heat that may cause tissue damage. In addition, there is a possibility that the balloon may rupture during expansion, causing leakage of material into the disc cavity and surrounding tissues, which may cause undesirable complications.
Yet another approach is disclosed in U.S. Pat. No. 5,865,846 to Bryan et al. in which a softer inner material is contained within a harder outer shell. The Bryan patent discloses the use of a resilient body made of two or more biocompatible materials wherein the soft inner portion of the body has a Shore D hardness of 30 and the harder outer portion has a Shore D hardness of 90. A similar approach using elastomers is described in U.S. Patent Publ. Appl. No. 2005/0119752A1 to Williams et al that discloses an artificial intervertebral disc fabricated of hydrogel, polyurethane, thermoplastic elastomers or other biocompatible materials wherein the softer inner nucleus portion has a Shore A hardness in the range of 20-70 and the harder outer portion has a Shore A hardness in the range of 35-90. While seemingly similar to the natural soft-hard combination of the disc nucleus and annulus, these implants still have the same approach and problems as the sandwiched metal and polymer implants.
Accordingly, there is a need for a nucleus disc replacement that addresses the shortcomings of the current approaches.